The present invention relates to a multilayer film composite for use in electrostatic or electrographic recording. This recording process is an important electronic non-impact printing technology which is now in wide use as a means for achieving both high speed recording and high quality images. Electrostatic printers or plotters are useful as output devices in computer-aided design, seismic recording, architectural design, and printed circuit design, among others.
Electrostatic recording is the process of producing an image in the form of an electrostatic charge pattern on a dielectric surface and subsequently developing that latent image by toning with oppositely charged black or colored powder, usually colloidally suspended in an insulating liquid.
In a typical writing or imaging process, a writing head, which contains two or more rows of densely spaced styli and a backplate or frontplate electrode, is selectively programmed by the plotter logic to place minute dot-spaced electrostatic charges in latent image form on the recording medium. This medium is designed to receive and hold an electrostatic charge pattern. After the latent image is electronically placed on the medium, the medium is exposed to a liquid toner. Black or colored particles suspended in the toner vehicle adhere to the medium only where a previous electrostatic charge was placed. Excess toner is removed from the medium by a vacuum channel or wiper bars and the medium is then dried by forced air, thereby fixing the image to the medium. This electronically produced print is often referred to as a hardcopy.
The most commonly used hardcopy media are paper, vellum and film. Each has its special requirements, and each has its special construction or design. This invention pertains to film and film-like surfaces for the electrostatic printing process.
Although various film recording media have been proposed for us with electrostatic recording plotters or printers, none of them has satisfied fully the substantial need in the art, particularly for the electrostatic recording devices such as the Benson (Oce Graphics) plotter. Verstec VS 3000 Series, 7000 Series, 8500 Series and 8500-HR Series plotters, CalComp 5700 and 5800 Series plotters and HP 7600 Series plotters. In fact, there are many deficiencies in the known products which have considerably limited their commercial utility.
The performance of conventional recording media is inherently moisture sensitive since the conductive layer of said media employs ionic moieties such as sulphonated polystyrene and dimethyl diallyl ammonium chloride. Thus, repeated images or lines (ghosting) and low density are obtained at humidities above 60% relative humidity (RH), and low image density results at humidities below 30%. This places an undesirable constraint on the operating environment. Water-soluble ionic moieties in the conductive layer cause bond failure and a resultant image layer breakup when the print is subjected to water. This water-sensitivity thus makes the conventional recording medium nonarchival.
Conventional recording media fail to provide fully satisfactory image quality, even al normal humidity conditions, i.e. 40-50% RH. Three main image defects which often are experienced in electrostatic recording are (1) flare which is randomly occurring bursts or explosions in plotter lines due to abnormal electrostatic discharge, (2) image breakup or dropouts which are irregular portions of missing image and (3) glitches which are irregular specks, zippers, or non-uniform images occurring in solid dark images due to irregularities in the dielectric surface of the media. Zippers, which are a common defect, are small horizontal lines which resemble a zipper seen in many recording films and are due to an electrical shorting of a stylus which causes loss of information across a small bank of multiplexed styli. While all three types of defects are undesirable, dropouts or image deletions are the most serious because of significant loss of information.
Furthermore, conventional recording media cannot transport reliably through the plotter, which results in inaccurate image rendition. This factor is very important where high accuracy plots are required such as in the aircraft industry. It is also important when multiple-color registration is required.
In spite of the many attempts that have been made to improve the qualities of the conventional recording media, none of the present commercial products is free from all these drawbacks.
The present inventors have discovered that a multi-layer composite comprising an image-receptive layer having electrical, surface profile and abrasivity characteristics within specific ranges, an electronically conductive layer possessing conductivity within a specific range over a wide range of ambient humidity, a supporting layer with high dimensional stability, and the ability to adhere to the adjacent layers, and, optionally, a layer which assists transport of the film through an electrostatic plotter can serve to overcome the above-mentioned drawbacks and perform in a manner that is superior to the films known in the art. This advance in the art results from finding that certain combinations of materials impart structural, electrical, chemical and physical properties to the resultant structure such that it is a superior electrostatic recording medium.
It is known in the art that an electrostatic recording medium must have a dielectric layer and a conductive layer, each conventionally defined, and, if neither provides the overall structural characteristics needed, a support layer. However, it also is known that this structure itself does not provide a good electrostatic recording medium. There is a major commercial need for a medium which not only satisfies this minimum requirement but which functions well under actual conditions of continuous use in a variety of devices at a wide range of humidities to provide an imaged product of high quality that can be used under practical conditions and that can be exposed to water without loss of archivability. Although this need is well known and a great deal of effort has been devoted to research and development in the field, a structure which satisfies such quality requirements and which can be produced economically has not been discovered previously. The balance of desirable properties achieved by certain structures against the consequent loss in other desirable properties has led to repeated failure of entirely rational design efforts because there are many requirements and they were not thoroughly understood either individually or in combination.
As a result of these and similar factors the current state of the art represents failure to discover the range of materials, structures and methods of producing them which can satisfy the clear need in the field. The present invention represents the discovery of such structures, the ranges of materials from which they can be assembled and methods for producing a high quality product.